The present invention relates to a measurement apparatus of optical transmission factor and, in particular, relates to such an apparatus which has no mechanical moving means. The present invention uses the particular structure of an optical beam splitter. The present invention has application, for instance in, a smoke indicator, a dust concentration indicator, a gas combustion control, etc.
The principle of measuring the optical transmission factor is to use a light source and a light detector. When the strength of the light is I.sub.0, and the strength of that light when the light passes the object which absorbs some of the light energy is I, the transmission factor T of that object is defined as, T=I/I.sub.0.
However, the direct application of that principle is not preferable, since it is subject to measurement error due to soil and/or dust on a window of the measurement apparatus.
A dust/soil free apparatus of a prior art is shown in our previous Japanese patent application No. 208550/82 as shown in FIGS. 1 and 2. In FIG. 1, the numerals 100 and 100a are measurement apparatus units, 102 and 102a are lamps, 104 and 104a are lenses, 106 and 106a are rotation mirrors, 108 and 108a are optical detectors, and 110 is an object for measuring transmission factor (t). When the mirrors 106 and 106a are at the position indicated by the solid line, then, the light from the lamp 102 is reflected by the mirror 106, and is applied to the detector 108, which provides the output voltage E.sub.1. Similarly, the light from the lamp 102a is reflected by the mirror 106a, and is applied to the detector 108a, which provides the output voltage E.sub.2. E.sub.1 and E.sub.2 are expressed as follows. EQU E.sub.1 =I.sub.1 m.sub.1 g.sub.1 EQU E.sub.2 =I.sub.2 m.sub.2 g.sub.2
where I.sub.1 and I.sub.2 are values showing the strength of the light of the lamps 102 and 102a, respectively, m.sub.1 and m.sub.2 are constants relating to the characteristics of the mirrors 106 and 106a, respectively, and g.sub.1 and g.sub.2 are also constants defined by the sensitivity of the detectors 108 and 108a, respectively.
Similarly, when the mirror 106 is at the position indicated by the dotted line position, and the mirror 106a is at the position which does not prevent a light beam, the light from the lamp 102a passes through the object 110 which has the transmission factor (t), then, is reflected by the mirror 106, and finally, detected by the detector 108, which provides the output voltage E.sub.1 '. Similarly, when the mirror 106a is at the dotted line position, and the other mirror 106 does not prevent the beam from the lamp 102, the light beam from the lamp 102 passes through the object 110, then, is reflected by the mirror 106a, and is detected by the detector 108a, which provides the output voltage E.sub.2 '. The values E.sub.1 ' and E.sub.2 ' are shown below. EQU E.sub.1 '=I.sub.2 (t)m.sub.1 g.sub.1 EQU E.sub.2 '=I.sub.1 (t)m.sub.2 g.sub.2
When th following ratio is calculated, the transmission factor (t) is obtained. ##EQU1## Since the values E.sub.1, E.sub.2, E.sub.1 ' and E.sub.2 ' are not measured at the same time, those values are measured on a time divisional basis according to the rotation of the mirrors.
However, the apparatus of FIG. 1 has the disadvantage that the mirrors must rotate, and the moving mirrors decrease the operational reliability of the apparatus.
FIG. 2 shows a prior at improvement of the apparatus of FIG. 1, and FIG. 2 uses a fixed polarization beam splitter, instead of a rotation mirror.
In FIG. 2, the numerals 1 and 11 are measuring apparatus units, and the object A is located between the apparatus units 1 and 11. Each of the apparatus units 1 and 11 has a light source 2 (12), an interference filter 3 (13), a first polarization beam splitter 4 (14), a first quarter wavelength plate 5 (15), a first reflection mirror 6 (16), a second polarization beam splitter 7 (17), a second quarter wavelength plate 8 (18), a second reflection mirror 9 (19), and a photo-detector 10 (20). The light source 2 (12) is a tungsten lamp or a light emission diode, and is controlled so that the lamps 2 and 12 provide light output alternately. Accordingly, when the lamp 2 is bright, the lamp 12 is dark. The light beam of the lamp 2 is applied to the interference filter 3, which restricts the range of the wavelength. Then, the beam is applied to the first beam splitter 4, in which the S polarization component is reflected, and the P polarization component passes through and is applied to the object A.
It should be appreciated that S polarization component is reflected by a beam splitter, and P polarization component passes through a beam splitter in the description of the present specification.
When the input beam is a circular polarization beam, the strength of the P polarization component is the same as that of the S polarization component. The reflected S polarization component passes the first quarter wavelength plate 5, reflected by the first reflection mirror 6, passes again the first quarter wavelength plate 5 which converts the polarization to the P polarization. Then, the beam is applied again to the first polarization beam splitter 4 and passes the same, and further is applied to the second polarization beam splitter 7 and passes the same. Then, the beam is applied to the second reflection mirror 9 through the second quarter wavelength plate 8, and is relfected by that mirror. The reflected beam passes again the second quarter wavelength plate 8 again, and is converted to the S polarization beam. The converted S polarization beam is reflected by the second polarization beam splitter 7, and is applied to the photodetector 10 which converts the optical power to the electrical signal E.sub.1. On the other hand, the P polarization beam which passes through the beam splitter 4 is applied to the second polarization beam splitter 17 through the object A, and passes that splitter, then, is applied to the photo-detector 20, which provides the output voltage E.sub.2 '.
When the light source 2 is dark, the light source 12 is bright. The light beam from the light source 12 is applied to the detectors 10 and 20, after passing along the dotted paths, and it is assumed that the detectors 10 and 20 provide the output signals, E.sub.1 ' and E.sub.2, respectively. The transmission factor (t) of the object (A) is obtained by calculating the following equation. ##EQU2##
The structure of FIGS. 1 and 2 has the advantage that the effect of soil and/or dust on a mirror or a polarization beam splitter is automatically compensated.
However, a polarization beam splitter which is a flat plate with a polarization film as shown in FIG. 2 has the disadvantage that the ratio of the reflected beam to the passed beam plus the reflected beam of P polarization component can not be small enough, and therefore, the calculated transmission factor is not accurate. The small ratio of the reflected beam to the passed beam plus the reflected beam of that polarization beam splitter comes from the narrow bandwidth from P component transfer wavelength to S component transfer wavelength of the characteristic curve between the wavelength and the transmissivity. That is to say, only the beam applied to a polarization film of a splitter with the Brewster angle is separated well enough.
The theoretical analysis when said ratio is small, that is to say, when a part of S component passes through a beam splitter, and/or when a part of P component reflects, is described below.
It is assumed in FIG. 2 that all the beam splitters 4, 7, 14 and 17 have the same characteristics as one another, and the leakage of each beam splitter is 10%. That is to say, it is assumed that 10% of S component passes through a beam splitter, and 10% of P component is reflected by a beam splitter, although said leakage is zero in an ideal beam splitter.
Concerning the light beam generated by the light source 2, the 90% of the P component passes through the beam splitter 4, the output of which is applied to the beam splitter 17 through the object A. Said beam splitter 17 also passes 90% of the input beam, and the output of the beam splitter 17 is applied to the detector 20, which provides the electrical signal E.sub.1 '=0.81(t) (=0.9.times.(t).times.0.9).
The first leakage of that P component is the 10% of P component, which is reflected by the beam splitter 4. That first leakage beam is applied to the photo-detector 10, through the quarter wavelength plate 5, the mirror 6, the quarter wavelength plate 5, the beam splitter 4 (the P component is converted to the S component in passing the quarter wavelength plate 5, and the converted S component passes through the beam splitter 4), the beam splitter 7, the quarter wavelength plate 8, the morror 9, the quarter wavelength plate 8, and the beam splitter 7 (the converted S component is converted again to the P component by the quarter wavelength plate 8, so the re-converted P component is reflected by the beam splitter 7). However, the amount of the leakage in that path is very small, and can be neglected in the analysis.
The second leakage of that P component is the 10% of P component reflected by the beam splitter 17. The second leakage beam is also applied to the detector 10, through the beam splitter 4, the object A, the beam splitter 17 (10%), the beam splitter 14, the object A, and the beam splitter 7. The electrical signal by that second leakage is 0.008(t.sup.2) (=0.9.times.(t).times.0.1.times.0.1.times.(t).times.0.9), and said signal is added to the value E.sub.1.
On the other hand, concerning the S component which provides the signal E.sub.1 in the detector 10, the normal optical path is, through the beam splitter 4(90%), the quarter wavelength plate 5, the mirror 6, the quarter wavelength plate 5, the beam splitter 4 (90%), the beam splitter 7(90%), the quarter wavelength plate 8, the mirror 9, the quarter wavelength plate 8, the beam splitter 7 (90%). Therefore, the signal E.sub.1 is 0.66 (=0.9.times.0.9.times.0.9.times.0.9).
The first leakage of that S component is caused by the beam splitter 4 which passes 10% of the S component. The 10% of the leaked S component is, through the object A, the beam splitter 17 (which reflects the S component), the beam splitter 14 (which also reflects the S component), the object A, the beam splitter 7, to the detector 10. The level by that leaksge path of the S component is 0.008(t.sup.2) (=0.1.times.(t).times.0.9.times.0.9.times.(t).times.0.1), and that level is added to the signal E.sub.1.
The second leakage of the S component is the reflection by the beam splitter 7, however, the amount of the second leakage is small, and can be neglected in the present analysis.
The third leakage of the S component is the path throught the beam splitter 4, the object A, the beam splitter 17, to the detector 20, which provides the signal 0.01(t) (=0.1.times.(t).times.0.1). That signal is added to the E.sub.1 '.
Accordingly, the level E.sub.1 and E.sub.1 ' considering the leakage is shown below. EQU E.sub.1 =0.66+0.008(t.sup.2)+0.008(t.sup.2)=0.66+0.016(t.sup.2) EQU E.sub.1 =0.81(t)+0.01(t)=0.82(t)
Similarly, the similar error occurs relating the beam generated by the light source 12 as follows. EQU E.sub.2 =0.66+0.016(t.sup.2) EQU E.sub.2 '=0.82(t)
Accordingly, the equation (1) is calculated as follows. ##EQU3## It should be appreceated that said value is preferably 1.0. Therefore, the 21% of error is caused by the leakage of ooptical beam by the beam splitters.